The muscle group known as the quadriceps--or "quads"--is made up of four different muscles that run along the front of the thigh bone. ("Quadriceps" means, literally, "four heads.") All four muscles act on the knee to extend the joint, straightening the leg.

Rectus Femoris

Origin: The anterior superior iliac spine (hip bone) and part of the ilium (the largest pelvic bone) near the acetabulum (the hip socket).

Insertion: The patellar (kneecap) tendon, which attaches the kneecap to the tibia (the shin bone).

Action: Extends the knee, straightening the leg. When the pelvis is fixed it also flexes the hip.

Vastus Intermedius

Origin: The upper two thirds of the front of the femur (the thigh bone).

Insertion: The outer edge of the patella.

Action: Extends the knee, straightening the leg.

Vastus Medialis

Origin: Along the inner edge of the linea aspera (a line that runs down the back of the femur). The muscle wraps around from back to front where it meets vastus intermedius to attach to its insertion.

Insertion: The lower medial (inner) edge of the patella.

Action: Extends the knee, straightening the leg. When the knee is bent it contributes to turning the tibia medially (inward).

Vastus Lateralis

Origin: Similarly to vastus medialis, it attacjes along the outer edge of the linea aspera at the back of the femur and wraps around to the front to meet vastus intermedius.

Insertion: The lower lateral (outer) edge of the patella.

Action: Extends the knee, straightening the leg. When the knee is bent it contributes to turning the tibia laterally (outward).


(The origin of a muscle is the end that it contracts towards, the insertion is the end that it contracts away from.)


SOURCES:

"Anatomy of Movement," Blandine Calais-Germain, 1993, Eastland Press, Seattle.
"Lower Extremity Muscle Atlas," Michael L. Richardson M.D., 1997, University of Washington,

Tags: Quadriceps